The present invention relates to an optical disk device for supporting a DVD-RAM disk. In particular, the present invention relates to a current-to-voltage conversion amplifier in an optical head, for converting a converted current fed from a detection photodiode to a voltage. Especially, the present invention relates to a concrete configuration of an amplifier which makes efficient use of the dynamic range of the amplifier and which is suitable for transmitting a reproduced signal to a signal processing circuit of a subsequent stage with a high signal-to-noise ratio.
The configuration of a head amplifier for reproducing a signal fed from a DVD-RAM disk is shown in FIG. 8A. A photodetector 80 functions also to detect a tracking error signal of a DVD-ROM disk. The photodetector 80 is divided into four sections. Each of photodetectors A, B, C and D is formed of a photodiode. Each photodetector converts incident light into a current. Each photodetector is connected to a current-to-voltage conversion amplifier 81. Its converted current is converted to a voltage. The resultant voltage is inputted to a summing amplifier 82. Thus, a reproduced signal corresponding to total incident light is obtained as a voltage signal.
The case where the current-to-voltage conversion amplifier 81 is formed by using an operational amplifier will now be described by referring to FIG. 8B. For the purpose of increasing the speed of light-to-current conversion, a reverse bias voltage (+Vcc) is applied to a cathode of a photodiode 83. An anode of the photodiode 83 is connected to an inverting input (xe2x88x92) of an operational amplifier 84. If there is no quantity of light incident on the photodiode 83, then no converted current flows, and an output voltage Vout of the operational amplifier 84 becomes equal to a reference voltage Vref applied to a non-inverting input (+) of the operational amplifier 84. In other words, Vref corresponds to zero incident light quantity. If reflected light from the disk is incident on the photodiode 83, then a converted current Ip flows out from the cathode of the photodiode 83. Since the inverting input (xe2x88x92) has very high input impedance, all of the current Ip flows through a gain resistor Rf. At this time, a voltage lower than the reference voltage Vref by Ipxc3x97Rf is obtained at an output terminal of the operational amplifier 84.
A track format of a DVD-RAM disk will now be described by referring to FIG. 9.
On a disk plate which is not illustrated, a groove having a wide width is formed in a spiral form.
There are two kinds of tracks called groove and land. Each track is further divided into units called sectors. The sectors are a groove sector 90 and a land sector 91. At the head of each sector, pre-pits 92 indicating address information are provided. The pre-formatted pits 92 are called PID (Physical IDentification) or Header field. In the DVD-RAM, the pre-formatted pits 92 are composed of four headers H1, H2, H3 and H4. As illustrated, two front headers H1 and H2 and two back headers H3 and H4 are disposed so as to be complementarily offset by half a track.
FIG. 10 conceptually shows an envelope of a reproduced signal obtained from a DVD-RAM disk having a capacity of, for example, 2.6 GB per single side, on the basis of an observation result. An arbitrary track of FIG. 9 is scanned with an optical spot. An optical spot position at that time is represented by the abscissa. A voltage level of a reproduced signal observed at this time is represented by the ordinate. A signal of a PID area 1000 becomes as shown in FIG. 10 because the quantity of light incident on the photodetector is lowered by a diffraction phenomenon of light at the pre-formatted pits. In a mirror section, the level of light incident on the photodetector becomes the highest because diffraction is not present. Hereafter, this level is referred to as mirror level. In a track composed of a groove and a track, the quantity of reflected light becomes lower than that of the mirror section 1001 as a result of diffraction of light performed by the groove. A signal level 1002 of a sector having no data recorded thereon corresponds to this. Hereafter, this signal level is referred to as land/groove level.
In order to describe a signal level on a recorded sector, a phase change medium used in DVD-RAMs will now be described briefly.
On the phase change medium, information is recorded by utilizing a phase transition phenomenon between two phases, i.e., a crystal phase and an amorphous phase. By converting a difference in refractive index between the crystal state and the amorphous state into a change in quantity of reflected light, recorded information is reproduced. In DVD-RAMs, there is employed such a material which is in a crystal state having a high reflection factor when no information is recorded, and which assumes an amorphous state having a lowered reflection factor when information is recorded. As shown in FIG. 10, therefore, a reproduced signal 1003 of a recorded sector has an amplitude in the darkness direction from the land/groove level.
On the other hand, an optical disk device using a magneto-optical (MO) medium has such a format that a data field recorded by magnetic marks follows a PID area composed of pre-formatted pits, in the same way as the DVD-RAM. While a pre-formatted pit signal is obtained as a change in light quantity caused by diffraction, however, a data signal is detected by converting rotation of the polarization plane of light caused by a recorded magnetic domain into a light quantity change. In a head amplifier of the magneto-optical disk device, therefore, the PID area signal and the data signal are obtained by using two different amplifiers. Furthermore, since the light quantity change caused by the recorded magnetic domain is slight, the gain of an amplifier circuit for the data field signal is typically set so as to be larger than that of an amplifier circuit for the PID area. Furthermore, since signals fed from different amplifier circuits are used, cross-talk caused mutually between signals must be removed. For that purpose, signals fed from two amplifier circuits are switched by a switch circuit according to whether the section is the PID area or the data field. A resultant single signal is subject to postprocessing. As a conventional technique of such a kind, JP-A-61-170938 can be mentioned.
The head amplifier is an amplifier for amplifying a detected signal obtained after light-current conversion. Thus the head amplifier has a role of assuring a sufficient signal quality and sending a resultant signal to a processing circuit of a subsequent stage. Therefore, the head amplifier needs to obtain a sufficiently large signal amplitude here. In other words, the head amplifier needs to have a large gain. In the DVD-RAM, however, the signal is obtained by performing the light-current conversion on light fed from the same photodetector, for both the PID area and the data field. In other words, there is a single amplifier circuit for converting a converted current to a voltage. As for a reproduced signal obtained from the single amplifier circuit, its signal level differs depending on whether the section is the PID area or the data field. The highest level in the PID area is the mirror level, whereas the highest level in the data field is the land/groove level. In the above described conventional technique of the magneto-optical (MO) disk device, therefore, the case where the reproduced signals of both the PID area and the data field can be obtained from a single detection system is not considered. The gain of the data field and the gain of the PID area cannot be set separately. As for the maximum gain of the data field, the gain of the head amplifier is limited so that the mirror level will not exceed the dynamic range of the head amplifier.
An object of the present invention is to provide a head amplifier capable of providing a detected output having such a signal amplitude as to make the signal-to-noise ratio of the data field compare favorably with that of the PID area.
Another object of the present invention is to provide an optical disk device having the above described head amplifier and having a reduced number of components.
The above described problem is caused by a premise that the signal is amplified with the same gain both for the PID area and the data field. Therefore, the above described problem is solved by amplifying the signal with different suitable gains respectively for the PID area and the data field.
Furthermore, when viewed from a different view, the above described problem is solved by adding an offset voltage corresponding to a difference between the mirror level and the land/groove level to the PID area, and increasing the gain by a quantity corresponding to the ratio of the mirror level to the land/groove level.
Other objects, features and advantages of the present invention will become apparent from the description of the embodiments of the invention taken in conjunction with the accompanying drawing.